Pulse width modulation multichannel transmitting system



D.- D. GRIEG PULSE WIDTH MODULATIO MULTICHANNKEL TRANSMITTING SYSTEM Filed June l2, 1944 2 SheetsShet l AZTIFA/EY Patented Aug. 5, 1947 PULSE WIDTH MODULATION MULTI- CHANNEL TRANSMITTING SYSTEM Donald D. Grieg, Forest Hills, N. Y., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application June 12, 1944, Serial No. 539,856

11 Claims.

This invention relates to multi-channel pulse--l type communication systems, and more particularly to multi-channel transmission systems and methods wherein the pulses of each channel differ from the pulses of other channels by a given pulse characteristic.

In the copending application, Serial No. 529,933 by E. Labin and D. D. Grieg, filed April '7, 1944, a multi-channel pulse communication system has been disclosed, wherein the pulses of the various channels are distinguished by various identifying pulse characteristics such, for example, as pulse width. The distinguishing pulse widths are obtained by means of pulse width Shaper circuits which require separate biasing and different parameters for each channel.

One of the objects of this invention is to provide an improved method and means for transmitting over a multi-channel communication L system.

Another object of the invention is to Provide improved methods and means for multi-channel pulse type communication wherein the pulses of the various channels are distinguishable by their Aof channels, wherein each channel includes an identical pulse generator which is time modulated by a signal, and the output of which withV respect to the amplitude of the pulses is made different for each channel in the output circuit thereof. The resulting channel pulse trains, being characterized by differences in amplitude, are then clipped at a common level, thereby resulting in pulses having a uniform amplitude but diering in width for the different channels.

The above and other objects and features of the invention will 'become more apparent upon consideration of the following detailed description to be read in connection with the accompanying drawings, in Which:

Fig. 1 is a block diagram of a multi-channel transmitting terminal according to my invention;

Fig. 2 is a schematic representation of a circuit illustrating a phase shifter and a time modulator-clipper which may be used in any of the channels of the circuit of Fig. 1;

Fig. 3 is a circuit in schematic form of a mixerclipper of the transmitter of Fig. 1; and

Fig, 4 is a graphical illustration of the operation of the transmitter of the invention,

Referring to Fig. 1, a multi-channel transmitter is shown having abase wave oscillator or generator l which supplies a basic wave directly to a modulator-clipper 2, and a base wave which is delayed or shifted in phase to modulator-clippers 3 and by means of the phasers or delay circuits 5 and E. It will :be of course understood that any additional number of channels as far as practicable, may be included in the transmitter. The modulator-clippers are seen to be supplied with modulating signals from signal sources 1, 8 and 9, respectively. The output of the modulators in each case is controlled by potentiometers I0, II and I2, which outputs are applied to a common mixer-clipper I3 feeding a transmission link, which may be either Wire or radio frequency, as desired. The modulator-clippers are shown biased from a common source 26.

Fig. 2 shows the circuit of one form of a pulse time modulator-clipper such as may be employed at 2, 3 or 4 in Fig. 1, together with a phase shifter of the character used at 5 and y5. Phase shifter 5 comprises a condenser resistor network C-R1, R2, the relative values of which determine the phase shift for each channel of the base wave applied thereto from the base Wave generator I, Fig. 1. After being shifted in its phase, the base wave is applied to the modulator-clipper at a primary coil I4 of a coupling transformer I5. The modulator circuit includes two secondary coils IS and I'I coupled to the control grids of two vacuum tubes I3 and i9 in push-pull arrangement similar to a full Wave rectiiier. Signal intelligence from signal sources l, 3 or 9 is applied to the modulator by means of the input connection 2) to primary coils 2| and 22 on the transformer I5. The modulator is effective in producing a so-called cusper type Wave which, for example, may be modulated in time by a signal, it is, of course understood that other forms of modulations, as for instance, with respect to pulse width may be recorded so The cusper Wave type output of the modulator is taken off a load resistor 23 and applied over a coupling condenser 24 to a clipper tube 25, the control' grid of which is biased to a negative potential for clipping purposes by means of a bias source 26. This bias source 26 is common to all the clipper tubes in the modulator-clipper circuits of all of the channels. (See also Fig. 1.) The clipper tube 25 is supplied with a plate potential B-lthrough a resistor 2l which may also serve as-an output amplitude varying potentiometer having an adjustable output terminal 28.

The output pulses clipped from the cusper wave of the modulator of each channel are preferably of negative polarity and are applied to the mixerclipper circuit of Fig. 3 which, as shown, is adapted to accommodate three channels. The

mixer clipper circuit comprises coupling elements 29 and 38 and a clipper tube 3| for each channel, which may be positively cathode-biased to cut on by means of a common bias source 32. It should be understood that the cutoi bias may also be obtained from a common (C) source connected to the grids of the individual mixer tubes. The outputs of all of the clipper tubes 3| may be obtained across a plate resistor 33 serving as a com mon load resistor for all of the tubes 3|. If desired, the pulses obtained across the resistor 33 may be further threshold clipped in the clipper 3S.

The operation of the transmitter of Fig. 1 will now be explained with reference to Figs. 2, 3 and 4.

The base Wave generator is effective in producing a base wave of the type shown at 34 in graph a. of Fig. 4. Three base Waves |B, 2B, 3B have been shown in graph a to indicate the relative phase position of the base waves as they are applied to the modulators 2, 3 and 4 respectively,

either directly as in the case of channel or by Way of .phase Shifters 5 and 6 for 4cliannels 2 and 3. The rectifying eiect of the modulator cincuit of Fig, 2 results in a cusper type wave 35, shown in graph b of Fig. 4. This cusper wave is similar in all channels and has therefore been shown only with respect to channel l. For the sake of clarity, the eiect of the modulation in time on the cusper Wave, or other forms of modulations by the signal has .been omitted in this instance, as this eiect is known in the art. It is pointed out that the linear portions of the cusps only are utilized during modulation and hence no change of waveshape y occurs for this portion during the modulation process. If further linearity is desired the sine Wave may be shaped into a triangular Wave form before application to the modulator. For transmission purposes, the cusps,` as at 36, are clipped from the cusper wave by the .properly biased clipper tube at a level 31 (graph b, Fig. 4) resulting in pulses identical for each. channel of the general type shown in graph c. The amplitude of these pulses, which is originally the same for all channels, may be controlled for each channel by means of the potentiometer-load resistor 2T, resulting impulses having different characteristic amplitudes, as exemplied .by pulses I, 2- and 3 of graph c. from their respective potentiometers 21, inverted in phase as at (Fig. 3*), are then clipped and amplied, all at the same level, by their respective mixer-clipper tube 3| (Fig. 3*). which is biased from the common bias source 32, at the clipping level 33 (graph c, Fig. 4). Since each of the pulses before being applied to the mixer i3, has a different amplitude, their being clipped at the common level 38 Will result in pulses having the same amplitude but vdifferent widths, character'- istic for each channel, as shown in graph dof Fig". 4, wherein the different Widths are indicated `by W1, W2, and W3 for the three channels respectively.

At 4|, Fig. 3, av pulsev as obtained from the mixer chpper tubes 3| is shown in detail, indicating its substantially trapezoid'al form. Since the pulses thus: produced differ from each other only in the top Width of the trapezoid, having a similar Width base, it may be desirable to further accentuate their width diiierence at either the transmission or the receiving terminal. If the former method is chosen, a possible solution is indicated in Fig. 3f, wherein the pulses from the mixer-.clippers 3|" The pulses of each channel, coming are submitted to a threshold clipping operation at a common level in the clipper 33, the clipper 39 may 4be of the .cathode-follower type, being productive of pulse portions as illustrated above the clipping level 42 of pulse 4|. The pulses thus applied to a transmission medium (not shown) will now differ in width for each channel more nearly as in graph d. It is understood, of course, that this threshold clipping may, if convenient, be performed at the receiving terminal of the system.

It is thus apparent that the use of means for producing channel pulses having various Widths as their distinguishing characteristic in the manner described, makes it possible for equipment to be used which is in part commonly controlled regardless of the number of channels employed, as Well as making for desirable simplicity in the individual channel equipment.

While I have described above the principles of my invention in connection with speciiic apparatus, it is to lbe clearly understood that this description is made only by way of example and not as a limitation on the scope of my invention as set forth in the objects of my invention and the accompanying claims.

I claim:

l. A method of multi-channel communication comprising producing a series of Ipulses for all oi a plurality of channels said pulses having at least one sloping edge, timing differently the pulses oi the 'different channels, modulating the timing characteristic of the pulses of each channel according to instantaneous Values of an intelligence signal, giving characteristic amplitude to the pulses of each of the channels, clipping the pulses of each channel thus characterized at a -level common to all channels, thereby obtaining pulses of equal amplitude and different characteristic Widths, and interleaving the diierent series of pulses together as a single train of pulses for transmission over a common transmitting medium. f

2. A method of multi-channel communication comprising producing a separate series of pulses having a characteristic amplitude for each of a plurality of channels and of a type having a sloping edge, timing differently the pulses of the different channels, modulating a characteristic whichV is a function of the pulse timing of the pulses of each channel according toY the values of an intelligence signal, clipping the pulses of each channel at a common level, and interleaving the different series of pulses together as a single train` of pulses for transmission over a common transmitting medium.

3. A method of multi-channel communication comprising producing a separate series of cusper waves for each of a plurality of channels, timing diierently said cusper waves of the different channels, clipping the said cusper waves at a common level to produce identical pulses for all channels, giving the pulses of each of the channels a characteristic amplitude, clipping' the pulses or? each channel thus characterized at; a level'v common to all channels, thereby obtaining pulses of equal amplitude and different characteristic Widths, and interleaving the diierent series of pulses together as a single train of pulses for transmission over a common transmitting medium.

4. A multi-channel communication system comprising separate means for producing a train of identical pulses for each 0f a pluralityV of channels said pulses having at least one sloping edge having a control common to said channels, means for timing differently the pulses of the different channels, means for time modulating said pulses of each train in accordance with s, signal, means in each channel for adjusting differently the amplitude of the pulses of the different channels, means for clipping said amplitude adjusted pulses at a level common to all channels, and means for interleaving the different trains of pulses together as a single train of pulses for transmission over a, common transmission medium.

5. A multi-channel communication system comprising, in combination, generator means for supplying a base Wave to each of a plurality of channels, means for timing differently the base Wave for each of the channels, means for producing from said base Wave in each channel a train of pulses identical for all channels, said pulses having at least one sloping edge, means for giving a characteristic amplitude to the pulses of each channel, means for clipping the pulses of each of said trains at a level common to all said channels, and means for interleaving the different trains of pulses together as a single train of pulses of different widths for transmission over a common transmission medium, said means for clipping including a clipper for each of the channels and a threshold clipper common to all said channels.

6. A multi-channel communication system comprising, in combination, generator means for supplying a base Wave to each of a plurality of channels, means for timing differently the base Wave for each of the channels, means for producing from said base Wave in each channel a train of pulses identical for all channels, said pulses having at least one sloping edge, means for giving a characteristic amplitude to the pulses of each channel, means for clipping the pulses of each of said trains at a, level common to all said channels, and means for interleaving the different trains of pulses together as a single train of pulses of different Widths for transmission over a common transmission medium, said means for clipping including a clipper having a common bias adjusted to cut off for each ofthe channels and a threshold clipper of the cathode follower type common to all said channels.

7. A multi-channel communication system comprising, in combination, generator means for supplying a base Wave to each of a plurality of channels, means for timing differently the base Wave for each of the channels, means for producing from said base wave in each channel a train of pulses identical for all channels, said pulses having at least one sloping edge, means for giving a characteristic amplitude to the pulses of each channel, means for clipping the pulses of each of said trains at a level common to all said channels, and means for interleaving the different trains of pulses together as a single train of pulses of different widths for transmission over a common transmission medium, said means for giving a characteristic amplitude comprising a potentiometer,

8. A multi-channel communication system comprising, in combination, generator means for supplying a base wave to each of a plurality of channels, means for timing differently the base wave for each of the channels, means for producing from said base wave in each channel `a train oi. pulses identical for all channels, said pulses having at least one sloping edge, means for giving a characteristic amplitude to the pulses of each channel, means for clipping the pulses of each of said trains at a level common to all said channels, and means for interleaving the different trains of pulses together as a single train of pulses of diiferent Widths for transmission over a common transmission medium, said `means for producing identical pulses including a clipper tube for each channel and a common control grid bias for all said clipper tubes.

9'. A multi-channel communication system comprising, in combination, generator means for supplying a base Wave to each of a plurality of channels, means for timing differently the base Wave for each of the channels, means for producing a cusper Wave from said base Wave in each channel, means for clipping the cusps of each of said channels, means for controlling said lastnamed means common to all channels, whereby identical pulses for all channels are obtained, means for giving a characteristic amplitude to the pulses of each channel, means for clipping the pulses of each of said trains at a level common to all said channels, and means for interleaving the different trains of pulses together as a single train of pulses of different widths for transmission over a common transmission medium.

10. A method of multi-channel communication comprising producing a separate series of pulses for each of a plurality of channels said pulses having at least one sloping edge, timing differently the pulses of the different channels, modulating a given characteristic of the pulses of each channel according to instantaneous values of an intelligence signal, giving the pulses of each o-f the channels a characteristic amplitude, clipping the pulses of each channel thus characterized at a level common to all channels, thereby obtaining pulses of equal amplitude and different characteristic widths, and interleaving the diierent series of pulses together as a single train of pulses for transmission over a common transmission medium.

11. A multi-channel communication system comprising separate means for producing a train of identical pulses for each of a plurality of channels said pulses having at least one sloping edge said means having a control common to said channels, means for timing differently the pulses of the different channels, means for modulating a given characteristic of said pulses of each train in accordance with a signal, means in each channel for adjusting differently the amplitude of the pulses of the dilerent channels, means for clipping said amplitude adjusted pulses at a level common to all channels, and means for interleaving the dilerent trains of pulses together as a single train of pulses for transmission over a common transmission medium.

DONALD D. GRIEG.

REFERENCES CETED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,048,081 Riggs July 21, 1936 1,655,543 Heising Jan. 10, 1928 2,266,194 Guanella Dec. 16, 1941 2,381,847 Ullrich Aug. 7, 1945 

